Chemical vapor deposition (CVD) processes are extensively used to grow layers of various thicknesses of metals, semiconductors, dielectrics and the like. A typical CVD process ordinarily requires that desired growth materials be attached to a ligand or volatile adduct that allows the transport of the desired species in the gas phase to a reaction zone in a reactor in which a substrate(s) is located. This complex molecule is commonly referred to as the precursor. Different materials possess different precursor structures.
Once in the reaction zone, a portion of the volatile precursor compound is decomposed, separating the volatile precursor compound from the non-volatile portion and leaving behind the desired solid deposit onto the substrate(s). Typically, the decomposition reaction is thermally driven; that is, the substrate is heated to a sufficiently high temperature such that when the volatile compound contacts the substrate, sufficient energy is made available to break the existing connectivity between the volatile ligand or adduct and the desired atom. The desired atom remains deposited on the substrate, while the volatile portion of the precursor gas is then exhausted from the reactor through an exit port. While thermal energy is one means for driving the CVD reaction, there are other process mechanisms that can be utilized for promulgating a suitable deposition.
Applicants have previously determined, as described in U.S. Pat. No. 6,482,649, entitled: Acoustic sensor for in-line continuous monitoring of gasses, the provision of at least one acoustic cell in a gas controller for controlling precursor delivery in a CVD reactor, such as a MOCVD (metalorganic Chemical Vapor Deposition) reactor and determining the composition of a binary gas mixture in order to ascertain the efficiency of the reactor. Such reactors were used, for example, for purposes of fabricating compound semiconductor devices for purposes of fiber-optic communications. The speed of sound of the gases flowing through the device are calculated wherein the device is operated in resonant mode. Inlet and outlet (exhaust) gas compositions can be ascertained from measured resonant frequency and speed of sound determinations. Utilizing the principles described in the '649 patent, Applicants have since commercialized a system referred to under the tradename, Composer Gas Controller.
At the present time, there is a resurgence of interest in MOCVD reactors, for example, for high brightness LED manufacturing processes. While the currently designed Composer device has been unsurpassed for its resolution and repeatability of measurements in its day, there are presently new challenges that the current device was not specifically designed to cope with. For example, TMIn precursor delivered by hydrogen carrier gas was typical in the past. At the present time, however, the delivery of Cp2Mg using nitrogen as a carrier gas is the biggest challenge. FIG. 1 graphically depicts each of the foregoing examples. In regard to same, the molecular weight ratio of the precursor to carrier gas is the most important factor in determining theoretical resolution of the device, as described in the above '649 patent, which is herein incorporated by reference in its entirety. Thus and in specifically transitioning from hydrogen carrier gas to nitrogen carrier gas, this key factor is diminished by a factor of fourteen.
It has been determined that this disadvantage can be significantly overcome, however, through various modifications to the acoustic resonator, as described herein, which now enables operation of the acoustic resonator at higher frequencies but without drastically altering its overall dimensions.
The constraints of commercially available acoustic transducers and microphones for introduction of acoustic energy, as well as the limitations of isolation diaphragms dictate that the device should operate in the frequency range of approximately 500 Hz to approximately 5000 Hz as a matter of practicality. In a compound resonator comprising multiple cavities of different sizes, such as those described in Applicants' prior U.S. Pat. No. 6,482,649, the admissible resonance frequencies are not harmonically related. For example, in the newly designed acoustic resonator (to be elaborated below), the theoretical resonance frequencies are 1208 Hz, 3948 Hz, 6827 Hz, 10161 Hz, etc. in nitrogen gas at room temperature. Although the resonance frequencies are different in different gases, however, the frequency ratios remain the same and are dictated by the geometry of the compound resonator.
Therefore a sound velocity sensor is herein provided, the sensor comprising a hermetic multi-chambered enclosure for containing flowing gases and mixtures of gases, means for acoustically exciting the contained flowing gases, and means for measuring acoustic energy transmitted over a fixed distance between a first sending end of said enclosure and a receiving end thereof. The speed of sound of the gases is determined by comparing the energy transmitted through the flowing gases at various frequencies so as to precisely determine the resonant frequency of the gases that are flowing through the enclosure. In accordance with the present design, the chambers of the enclosure include internal transition shapes therebetween for optimizing the transmission of acoustic energy through the flowing gases and for enhancing one or more additional resonant modes at higher useful frequencies.
The transition shapes used in connection with the herein described sound velocity sensor can be at least one of parabolic, hyperbolic, linear and exponential in nature or other suitable continuous shape to facilitate the transmission of acoustic energy and to minimize acoustic impedance loss in the chambered enclosure of the sensor.
The additional resonant modes achievable by the presently described sensor are within the range where readily available sending, receiving transducers and isolation diaphragms may be produced and the devices fail to have self-resonant modes of consequence.
According to one preferred version, the fundamental resonant frequency and additional resonant frequencies lie between about 400 Hz and about 6000 Hz.
The temperature of the sensor housing and the gases flowing therethrough are preferably controlled to arbitrarily precise levels.
The determined resonant frequencies can be used, for example, for deriving the composition of a binary gas mixture used in a reactor such as those utilizing MOCVD processes.
Advantageously, the sensor improvements discussed above enhance the stability and sensitivity of the resonant frequency determinations, as well as the resulting velocity of sound calculations obtained.
These and other features and advantages will be readily apparent from the following Detailed Description, which should be read in conjunction with the accompanying drawings.